The presidential hopeful and campaign manager John Podesta tried to unearth the truth about the bizarre case, but were met with a wall of silence after the authorities claimed the files had simply “disappeared”.

Mr Podesta, who once tweeted his greatest regret was not getting the disclosure of all top-secret UFO files held by the US Government, revealed the pair had tried to get to the bottom of the as yet unidentified case.

Mr Podesta said he helped Mrs Clinton use the Freedom of Information Act to request transparency and files on the UFO case in question, but they were stonewalled.

The campaign manager gave few details on the incident that triggered the FOIA request, saying it involved a crash of what he described as a possible UFO or Soviet device.

Many UFO buffs believe it may have been concerning the legendary Roswell case in the New Mexico tester in July 1947, when the US military announced it had found a crashed flying saucer, only to retract it the following day with a statement it was a top-secret military weather balloon.

The case has remained one of the world’s biggest unsolved UFO cases to this day, and the subject of numerous hoaxes.

Mr Podesta said: “I worked with her and filed an FOIA case on it.

“The files had disappeared but it was clear there’d been some investigation by the Air Force.”

Ironically, Mrs Clinton has previously been painted as an enemy of transparency due to the long-running saga over her use of a private email server during her tenure as Secretary of State.

Several lawsuits under the Freedom of Information Act (FOIA), were used to release the emails.

Mr Podesta conceded: “I think it’s definitely hurt her.

“The decision at the outset to use a private email system was, in retrospect, the wrong one. She did it out of convenience but it’s been anything but.”

Mrs Clinton and Mr Podesta have been saying she will strive to open up any secret UFO files if she makes it into the White House.

But in March, Express.co.uk revealed the pair had been involved in a secret “Rockefeller Initiative” to get the truth out there in the 1990s, but have refused to talk about what happened ever since.